In producing a rubber-modified polymer, conventionally, as a raw material, a rubber dissolved, for example, in styrene, in the case of the production of an impact-resistant polystyrene, or in styrene and acrylonitrile, in the case of the production of an ABS resin, in the presence or absence of a solvent, such as benzene, ethylbenzene, toluene, and xylene, is fed to a reactor.
Conventionally, to dissolve a rubber in the above liquid, a predetermined quantity of the rubber and the above liquid that would attain a desired concentration of the rubber, was fed to a dissolution tank, and the rubber was dissolved completely in the liquid in the tank. Generally, as one method for dissolving a rubber completely, a method was employed wherein a rubber was dissolved in a rubber dissolution tank batch-wise.
As a further method proposed, a method disclosed in JP-A-4-130111 ("JP-A" means unexamined published Japanese patent application) can be mentioned. In the method disclosed in this application, a wet grinding apparatus was positioned between two rubber dissolution tanks, to make a rubber readily dissolved.
The concept of the dissolution rate of a rubber in the above prior art dissolution method is illustrated by the curve b in FIG. 2. As is shown by the curve b, in these methods, the rate at which a rubber is dissolved in a monomer liquid or a solvent gradually decreases as the amount of undissolved rubber decreases according to increase of concentration of the rubber in the rubber solution by the dissolution of the rubber. Therefore it requires a considerable period of time for a rubber to be dissolved completely. Accordingly, the volume of the dissolution tank was large.
Therefore, the prior techniques were accompanied by the following problems to be solved:
(1) In the method wherein a rubber is dissolved in a rubber dissolution tank batch-wise, the dissolution tank is large and the dissolution efficiency of a rubber is not good. PA1 (2) In the method disclosed in JP-A-4-130111 in which two dissolution tanks are required, although the dissolution tank in the first half can be made smaller than the dissolution tank described in the above (1), the dissolution efficiency of rubber is not good in the second dissolution tank in the second half. Though the dissolution by the second dissolution tank is improved a little in dissolution rate, it is same as batch-wise dissolving in which the rubber is to be dissolved completely. Specifically, the concentration of a rubber in the second dissolution tank is required to be the concentration of the raw material for producing rubber-modified polymer with the rubber being dissolved and undissolved in a styrene-type monomer and a solvent including an undissolved rubber. Thus, along with rise of rubber concentration, the dissolving velocity of rubber becomes slow due to decrease of the undissolved rubber. As a result thereof, it is required to allow a longer residence time, so as to allow a small quantity of an undissolved rubber to be dissolved, and therefore the volumetric efficiency of the dissolution tank becomes poor and therefore it is difficult to make the total volume of the first and second dissolution tanks extremely small. Further, consequently, when the rubber concentration desired is varied during continuous operation, it takes a longer time for the final dissolution tank to reach the desired rubber concentration.